index.css

* {
  border: 0;
  margin: 0;
  outline: 0;
  padding: 0;
}

body {
  font:
    14px/22px '-apple-system',
    'Helvetica Neue',
    Helvetica,
    Arial,
    sans-serif;
  position: absolute;
  width: 100%;
}

.buttons {
  align-items: center;
  position: absolute;
  display: flex;
  justify-content: end;
  top: 50px;
  right: 0;
  padding: 5px;
}

#errorIcon {
  color: red;
}

.button {
  background: transparent;
  border-radius: 4px;
  color: #333;
  font-size: 12px;
  height: 30px;
  line-height: 30px;
  text-align: center;
  touch-action: manipulation;
  user-select: none;
  width: 40px;
}

.button:hover {
  background: #eee;
  color: #111;
}

#app {
  display: flex;
  height: 100vh;
}

#leftInput,
#rightOutput {
  position: relative;
  display: flex;
  flex-grow: 1;
  flex-basis: 0;
  flex-direction: column;
}

#rightOutput {
  border-left: 1px solid #cccccc;
}

textarea {
  border-top: 1px solid #cccccc;
  box-sizing: border-box;
  font-size: 15px;
  height: 100%;
  padding: 10px;
  resize: none;
  flex-grow: 1;
}

#rightOutput textarea {
  background: #eeeeee;
}

h2 {
  font-weight: normal;
  line-height: 50px;
  padding: 0 10px;
  text-align: center;
}

.dropdown {
  position: relative;
}

.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 200px;
  box-shadow: 0 8px 16px 0 rgba(0, 0, 0, 0.2);
  padding: 12px 16px;
  right: 0;
  z-index: 1;
  user-select: none;
}

.dropdown:hover .dropdown-content {
  display: block;
}